Functional diversity and climate change: effects on the invasibility of macroalgal assemblages

Climate-driven and biodiversity effects on the structure and functioning of ecosystems are increasingly studied as multiple stressors, which subsequently may influence species invasions. We used a mesocosm experiment to test how increases in temperature and CO2 partial pressure (pCO2) interact with functional diversity of resident macroalgal assemblages and affect the invasion success of the non-indigenous macroalga Sargassum muticum. Early settlement of S. muticum germlings was assessed in the laboratory under common environmental conditions across three monocultures and a polyculture of functional groups of native macroalgae, which had previously grown for 3 weeks under crossed treatments of temperature and pCO2. Functional diversity was a key driver shaping early settlement of the invader, with significant identity and richness effects: higher settlement occurred in low-diversity and low-stature assemblages, even after accounting for treatment biomass. Overall, early survivorship of settled germlings responded to an interaction of temperature and pCO2 treatments, with survivorship enhanced in one treatment (high pCO2 at ambient Temperature) after 3 days, and reduced in another (ambient pCO2 at high Temperature) after 10 days, although size was enhanced in this same treatment. After 6 months in the field, legacy effects of laboratory treatments remained, with S. muticum reaching higher cover in most assemblages previously subjected to ambient pCO2, but ephemeral green algae appearing disproportionately after elevated-pCO2 treatment. These results caution that invasion outcomes may change at multiple points in the life cycle under higher-CO2, higher-temperature conditions, in addition to supporting a role for intact, functionally diverse assemblages in limiting invader colonization.